Hi Dave,
I've heard several stories of mantis living 'happily' with tank mates for several months then...................!!!! as someones else says it ends not with a bang, not with a wimper but a thwack.....
There is a shop in the UK that has a very large peacock smasher in a species nano. The nano had a big flat worm problem so the owner put a six line in. The wrasse ate all the flat worm over a period of a few months. When they had all gone the peacock ate him. Hardly fair after the spring clean he'd done.